As methods for developing an electrostatic latent image formed on an electrostatic latent image retainer (photoreceptor) in an image forming apparatus employing electrophotography there have heretofore been known various methods depending on the kind of the developer used. Among these methods is a method which comprises allowing a developer supported on a developer carrier to come into contact with a photoreceptor or allowing the developer to fly onto the photoreceptor to effect development. As the surface layer of the developer carrier for carrying the developer there have been proposed various resin layers.
For example, JP-A-63-311367 (The term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses a developer carrier which comprises as a surface layer an external layer having a predetermined relationship between the volume intrinsic resistivity (1.times.10.sup.6 to 1.times.10.sup.13 .OMEGA..multidot.cm) and the thickness to enhance gradation and fine line/halftone dot gradation reproducibility. JP-A-4-166864 discloses a developer carrier which comprises as a surface layer a thin layer containing a positively-chargeable particulate resin and employs as a developer a negatively-chargeable (one-component) magnetic toner to inhibit the drop of the initial image density due to broad distribution of charge in the developer at the starting of the development apparatus. Further, JP-A-4-246676 discloses a developer carrier which has been polished so that the average roughness pitch Sm on the surface film layer and the central line average roughness Ra are restricted to inhibit a phenomenon that the same pattern images obtained by repeated printing show a reduced density partly along the feed direction of the paper.
However, it can hardly be said that the conventional developer carriers can be put into practical use. These conventional developer carriers have the following disadvantages.
In particular, the developer carrier disclosed in JPA-63-311367 is disadvantageous in that the external layer has a high resistivity. Thus, electric charge remains on the area of the external layer from which the developer has migrated to the photoreceptor during development. This area has more electric charge than other areas. The developer which is subsequently supported on this area is adsorbed by the external layer too strongly to migrate to the photoreceptor. This results in a so-called development ghost that deteriorates the image quality. In some detail, the newly developed toner image shows a reduced density at the area corresponding to early developed toner images.
The developer carrier disclosed in JP-A-4-166864 is disadvantageous in that the dispersion of the positively-chargeable particulate resin is locally ununiform, making the resistivity of the thin layer ununiform. Thus, the thin layer having a reduced resistivity allows easy passage of electric current, causing the generation of bias leak across this area and the photoreceptor. This can destroy the surface of the photoreceptor. If the photoreceptor has defects such as pinhole on the surface thereof, the resulting image has coarse grains and hence a reduced quality. Further, if a negatively-chargeable magnetic toner comprising as a developer a silica capable of being strongly negatively charged is used, development ghost, which shows the history of print pattern, can occur on the development carrier.
The development carrier disclosed in JP-A-4-246676 is disadvantageous in that the average roughness pitch Sm on the surface film layer is too small. Thus, the developer grains are caught by the grooves on the film layer and thus can hardly migrate to the photoreceptor. Accordingly, the developer grains on the developer grains caught by the grooves are insufficiently charged, causing an image density drop.
As mentioned above, the conventional developer carriers coated with a resin layer as a surface layer are disadvantageous in that they are apt to development ghost or bias leak due to the excess or shortage of charge in the developer provided by frictional charging, the use of a negative-working developer comprising an external additive capable of being strongly negatively charged or the generation of discharge between the resin layer and the surface of the photoreceptor attributed to the ununiformity of the resistivity of the resin layer.